This invention relates to methods and apparatus for determining the magnitude of pulses, and in particular for determining the magnitude of a pulse in a signal having both positive and negative polarity pulses.
The telecommunications Plesiochronous Digital Hierarchy (PDH) is widely used to transmit digital information (e.g. digitised telephone calls) along telephone cables at any of a number of standardised data rates. For rates of 44.768 MHz and below a ternary (three-level) signal waveform is used. In this waveform a logic 1 symbol is indicated by the presence of a pulse (relative to a predefined timebase) and a logic 0 symbol is indicated by the absence of a pulse. In general successive logic 1 symbols are represented by respective pulses of opposite polarity to one another. Thus a data signal is made up of positive and negative pulses indicating logic 1 symbols and gaps indicating logic 0 symbols.
Test equipment for monitoring the status of a PDH signal typically reports parameters such as frequency, jitter and voltage levels. The voltage levels reported are peak-to-peak, positive pulse magnitude and negative pulse magnitude. In a healthy signal the positive and negative pulse magnitudes are substantially identical. However, one possible indication of the presence of a fault is the occurrence of opposite-polarity pulse sizes which are not the same, and for this reason a measurement of the individual positive (+V) and negative (xe2x88x92V) pulse sizes is supplied.
These voltage measurements are usually made with analogue-to-digital converters (ADCs) which provide a digital output indicative of the analogue voltage at a particular instant. Low-cost ADCs are preferred for this purpose for reasons of economy, but they have a single supply voltage and polarity, and hence cannot directly accept the bipolar ternary PDH signal. Accordingly the bipolar signal is ac-coupled to the input of the ADC so that the dc offset between the zero voltage rail of the ADC and the maximum negative voltage of the incoming signal is removed. The average voltage of the ac-coupled input signal is regarded as representing the zero voltage level of the original bipolar signal, and this is valid if the signal is healthy.
Current test equipment uses the average voltage as the zero reference point relative to which the positive and negative voltage magnitudes are measured. However, if the positive and negative pulses are asymmetric in magnitude then the average voltage of the ac-coupled signal no longer corresponds to the zero voltage level. This gives rise to strange effects. For example, if the negative pulse magnitude varies, whilst the positive pulse magnitude does not, then the measured value for the positive voltage nonetheless also changes. This arises because of the variation in the average voltage resulting from the increase or decrease in the voltage excursions below the zero level.
The inventors hereof have realised that it is desirable instead to measure from the actual signal zero level. Even if the negative pulse magnitude is varied, the voltage range from the zero level to the positive pulse maximum stays the same. By measuring from the zero level of the signal, a truer indication of the magnitude of the positive and negative pulses is obtained.
According to one aspect of this invention there is provided a method of determining the magnitude of a pulse in a signal having both positive and negative polarity pulses, comprising the steps of:
measuring the magnitude of multiple samples of the signal;
determining from a plurality of the sample magnitudes a reference voltage level for the signal; and
determining the magnitude of a pulse in the signal from the sample magnitudes and the reference voltage level.
The reference level may be determined from a median value of the plurality of sample magnitudes. In this case the method may be implemented by: counting occurrence of different sample magnitudes during a measurement interval; aggregating counts for different sample magnitudes taken in order at the end of the measurement interval, and comparing the aggregated count to a threshold value representing the median value; and selecting the sample magnitude whose corresponding count causes the aggregated count to attain or exceed the threshold value, as indicative of the reference level.
According to another aspect of this invention there is provided apparatus for determining the magnitude of a pulse in a signal having both positive and negative polarity pulses, comprising:
measuring means for measuring the magnitude of multiple samples of the signal;
reference determining means for determining from a plurality of the sample magnitudes a reference level for the signal; and
magnitude determining means for determining the magnitude of a pulse in the signal from the sample magnitudes and the reference level.
The apparatus may further comprise: a counter for counting occurrence of different sample magnitudes during a measurement interval; an adder for aggregating the counts for different sample magnitudes taken in order at the end of the measurement interval; a comparator for comparing the aggregated count to a threshold value representing the median value; and a store for storing as the reference level the sample magnitude whose corresponding count causes the aggregated count to attain or exceed the threshold value.